Zinc plating



Patented May 18, 1937 UNITED STATES PATENT OFFICE ZINC PLATENG NoDrawing. Application April 4, ,1935, Serial No. 14,589

19 Claims.

This invention relates to the electrodeposition of zinc, andis'particularly directed to processes wherein a bright, mirror-like zincdeposit is plated from cyanide-zinc baths which contain substantialamounts of a soluble compound of a metal 01 group VI sub-group 1 of thePeriodic System, and which, in addition, may also contain a metal ofgroup VII sub-group 1 a metal of group VIII series 4, and organicaddition ll) agents. This invention, moreover, is also directed to thenovel zinc deposits obtained by such processes.

The electrodeposition of zinc, or electrogalvanizing, has been ratherextensively employed, be-

cause electrodeposited zinc coatings, in addition to their low cost,display many characteristics which cause them to be particularlydesirable as protective finishes. While zinc is not. itself veryresistant to corrosion, it does not accelerate 29 the corrosion of ironor steel as do such metals as copper, nickel, and chromium. On thecontrary, zinc, being higher in the electremotive series, will protectiron or steel against rust even after appreciable areas of the basemetal are exposed. Despite their numerous advantages over many commonlyused coating materials, electrodeposited zinc coatings have not enjoyedthe use they deserve because they do not possess and do not retain apleasing appearance and, conse- ;m quently, for many purposes they arenot acceptable.

The known methods of electrodepositing zinc almost invariably result indark colored or dull plates and, evens-when the deposits at first are37, fairly satisfactory, they soon become dark and discolored. .The poorappearance of electrodeposited zinc coatings has limited their usealmost exclusively to purely protective applications, and

those working in the art have turned to other in protective materialswhen it was desired to produce a finish ,of pleasing appearance.

The electrodeposition of zinc has ordinarily been accomplished by theuse of either an acidzinc bath or a cyanide-zinc bath. With neither 4.1of these baths has it been possible to obtain satisfactory smooth andbright deposits, but the acidzinc bath is most commonly used because itleads to a brighter deposit with a better color than does thecyanide-zinc bath.

50 While, under favorable conditions, the deposits obtained fromacid-zinc baths are relatively white, the deposits are still none toosatisfactory because of their relatively coarse crystalline structure.

55 Numerous attempts have been made to improve the character of zincdeposits obtained from acid baths, and many addition agents, such asglycerine, dextrin, gum tragacanth, licorice, naphthalene compounds, andaluminum compounds, have been used in conjunction therewith. While the5' use of addition agents improved the character of the deposits to someextent, the results were still none too satisfactory.

In addition to the fact that acid-zinc baths do not produce satisfactorydeposits, there are nu- 10 merous other disadvantages attendant upontheir use. For one thing, acid-zinc baths have very poor throwing power,and it'is exceedingly difficult satisfactorily to plate irregularlyshaped objects. Another disadvantage of acid-zinc baths is their lowcathode efiiciency. As zinc is above hydrogen in the electromotive forceseries of metals, it is theoretically impossible to deposit zinc fromacid solutions, but, of course, the rather great overvoltage of hydrogendoes per- 20 mit some zinc deposition. Concurrently with the depositionof zinc, however, there is a very considerable evolution of hydrogen.

While the deposits obtained from cyanide-zinc baths are poor inappearance, they have a relatively fine crystalline structure. A fewaddition agents, such as alum, gum arable, and fluorides, have beentried in cyanide-zinc baths, but the results obtained were none toosatisfactory. Aside from the poor appearance of deposits obtainabletherefrom, cyanide-zinc baths have a number of advantageouscharacteristics. They have good throwing power, and it is thereforepossible to deposit a relatively uniform zinc coating on irregularlyshaped and recessed articles. Cyanide-zinc baths, moreover, have arelatively high cathode efliciency which, of course, is veryadvantageous because the electric current applied to the bath isexpended less upon the evolution of hydrogen, and more upon thedeposition of zinc.

Despite the advantages of cyanide-zinc baths, they have 'not been muchused by the art because of the poor appearance of zinc depositsobtainable therefrom. Regardless of the disadvantages in operation ofacid-zinc baths, they have been favored by those working in the artbecause of the somewhat better appearing deposits obtainable by theiruse.

It is an object oimy invention to provide processes by means of whichbright, smooth zinc electrodeposits can be obtained. A further object ofmy invention is to provide processes by means of which the advantages ofcyanide-zinc baths can be realized, and by means of which, at the sametime,'deposits of pleasing appearance can be produced. A still furtherobject of my invention is to provide electroplating baths which arecharacterized by good throwing power, and which will produce bright zincdeposits over a relatively extended range of current densities. A stillfurther object of my invention is to provide processes which producezinc deposits that will respond to dilute oxidizing bright dips. Furtherobjects of my invention will become apparent hereinafter.

I accomplish my objects, briefly, by the use of cyanide-zinc baths towhich has been added a metal of group VI sub-group 1, of the PeriodicSystem. In addition to using metals of group VI, I may use metalsofgroup VII sub-group 1, and metals of group VIII series 4. My novelbaths may also contain a suitable organic'addition agent. The metalcontent of these baths may be maintained by using zinc anodes containingan adequate amount of the metal addition agent or agents.

Considering the features of my invention in more detail, it is firstnoted that a' cyanide-zinc bath must be employed if satisfactory resultsare to be obtained. Numerous specific examples of typical cyanide-zincbaths are given hereinafter, but it will be understood that'theprinciples of my invention are applicable to any of the cyanide-zincbaths already known in the art.

The principal brightening agent of my invention is comprised of a metalof group VI subgroup 1 of Mendelejefls periodic arrangement of theelements. The metals of this group, molybdenum, chromium, tungsten, anduranium, display a profound effect upon the character of a zincelectrodeposlt. The metals, of course, must be present in the form of acompound which is soluble in the bath. They may be added to a bath, forinstance, as a molybdate, chromate, tungstate, or uranate of sodium orpotassium. While I may satisfactorily use any one or more of the membersof this group, I have found molybdenum the most satisfactory and,accordingly, I prefer to use it as a metallic brightening agent.

The efficacy of metals of group VI sub-group 1 may be enhanced byemploying in conjunction therewith a metal of group VII sub-group 1and/or a metal of group VIII series 4 of Mendele- Jefis Periodic System.The metals of group VII sub-group 1 tried are manganese and rhenium, andthe metals of group VIII series l tried are iron, cobalt, and nickel.There is apparently some synergetic action between the metals of groupVI and group VII or VIII series 4 which leads to results superior tothose attributable to the individual action of themetals. The metals ofgroup VII sub-group 1, or group VIII series 4, are, of course, added tothe bath in the form of such alkali or cyanide soluble compounds asmanganese sulfate, potassium perrhenate, potassium ferrocyanide, cobaltsulfate, nickel sulfate, cobalt oxide, and nickel oxide. The users!metals ofgroupVIIsub-group 1 andgroupmseries-tseparnteandapartfromthemetalsof gmupvl sub-group I, constitutes no partof the plums invention.

In macaw the metai brightening agents. the bath may contain suitableorganic addition agents of known type. For instance, such additionagents as glue, dextrin, sulfite cellulose waste. licorice, gum arabic,gum tragacanth, or fur-fund may be used. If it is desired to use anorganic addition agent, I prefer to use thiourea, or a substitutedthiourea. I may use an alkyl or aryl thiourea which is soluble in thebath, such as phenylthiourea or diphenylthiourea-disulfonic acid. It isto be understood that the use of thioureas constitutes no part of thepresent invention apart from the use of metals of group VI sub-group 1.

Manganese displays a considerable tendency to precipitate from the bath,and I have found that the addition of a small amount of a soluble ironcompound tends to keep the manganese in solution. When used inconjunction with manganese, iron, then, acts both to keep the manganesein solution and to brighten the zinc deposit. A similar solubilizingtendency is exhibited on other metals of group VI sub-group 1 by othermetals of group VIII series 4. To facilitate the deposition of zinc oncast iron articles, it may also be found desirable to employ a smallamount of mercury in known manner.

The zinc. deposits obtained according to the processes of my inventionare not pure metallic zinc, but, rather, are alloys of zinc with smallamounts of whatever metallic brightening agents are used. Whenmolybdenum was used as the brightening agent, an amount in excess offive hundredths of one per cent of molybdenum was present in the zincdeposit. A much smaller amount of manganese was deposited with the zinc.It seems probable that the presence of a metal of group VI sub-group 1renders the zinc deposit somewhat more resistant to corrosion,

and, as the various novel characteristics of my novel zinc deposits areprobably at least in part attributable to the presence of suchbrighteners, I regard the alloys of zinc with a metal of group VIsub-group 1, or with both such a metal and a metal of group VIIsub-group 1, as an article of my invention.

In view of the fact that the metal brighteners are deposited with thezinc, it is necessary to add such metals to the bath from time to time.In-

stead of adding these metallic brighteners in the form of their solublecompounds, the bath may be constantly replenished by using an anodewhich contains an adequate amount of the metallic brightener.

While the zinc deposits produced according to the processes of myinvention are very bright and smooth, it. may sometimes be desirablefurther to brighten them and, more particularly, to render them passiveto finger'staining-and to other stains which might result from handling.The deposits produced according to the processes of my invention respondvery satisfactorily to the action of oxidizing bright dip solutions, and

, by bright dipping a deposit in a mildly oxidizing solution, thedeposit will be rendered brighter, any slight color will be removed, andthe deposit will be rendered passive. v

My novel zinc deposits are preferably bright dipped with an acidichydrogen peroxide'solu tion. Such a solution may be made byrnixingsulfuric acid with hydrogen peroxide, the ratio OfBzOz.toHz;beinglbout48to.l,to4tolby weight. ,Mom spleciflcgllyJ-peeferthatthemtbWhile thel fist. he obtainedbyfthe' use of an acidic mama so'hztion'other bright dips such as an acidic chromic acid solu-.

- of hydrogen peroxide and sulfuric acid. The

Example I A cyanide-zinc bath containing a metallic brightening agentfrom group VI sub-group 1 was made up as follows:

Grams per liter Zinc oxide (ZnO) 45. Sodium hydroxide (NaOH) 38 Sodiumcyanide (NaCN) 100 Molybdenum trioxide (MoOa) -a 7 Example I! A zincplating bath containing a metal of group VII sub-group 1, as well as ametal of group VI sub-group 1, was made up with the followingcomposition:

Grams per liter Zinc oxide (ZnO) 45 Sodium hydroxide (NaOH) 38 Sodiumcyanide (NaCN) 100 Molybdenum trioxide (M003) 3 Manganese sulfate(MnSOuiHzO) 1 Excellent results were obtained by the use of this bath.The deposit had a bright mirror-like surface with a slightly bluish hue.The molybdenum trioxide corresponds to about two grams per liter ofmolybdenum, and the manganese sul-.

fate corresponds to about twenty-five hundredths of a gram per liter ofmanganese.

Example III As has been noted above, organic addition agents mayadvantageously be used with the baths of my invention. A cyanide-zincbath containing an organic addition agent as vwell as a metal of groupVI sub-group 1 was made up with the following composition:

Grams per liter Zinc oxide (ZnO) 45 Sodium hydroxide (NaOH) 34 Sodiumcyanide (NaCN) 100 Ammonium sulfate ((NHOzSOa) 18 Molybdenum trioxide(MoOs) 4 Furfural 3 Excellent results were obtained with the bath ofthis example. The molybdenum trioxide used corresponds to about two andseven-tenths grams per liter of molybdenum.

Example 1V Another bath containing an organic addition agent was made upwith the following composition:

Grains per liter Zinc oxide (ZnO) 45 Sodium hydroxide (NaOH) 38' Sodiumcyanide (NaCN) 4. Molybdenum trioxide (MoOa) 1 Thiourea 10 Anexceedingly smooth zinc deposit was obtained by the use of this bath,but the deposit had a slightly yellowish hue. To obtain a perfectlyclear and bright. finish, the articles plated in this bath were treated,briefly, with a bright dip made up zinc oxide (ZnO) ratio of H202 toH2804 in the bright dip was about 16 to 1 by weight. It is noted thatthe molybdenum trioxide used corresponds to about sixtyseven hundredthsof a gram per liter of molybdenum.

Example V As an example showing the use 'of another 'metal of group VIsub-group 1, chromium, a bath of the following composition is given:

Grams per liter Sodium hydroxide (NaOH) 38. Sodium cyanide (NaCN) 100.Sodium chromate (Na:CrO4-10Hz0) 0.25

Thiourea 10.

- Quite satisfactory results were obtained with this bath. The thiourea,of course, can be omitted, or other organic addition agents may be usedin lieu thereof. It is noted that the chromate used is equivalent toabout thirty-eight thousandths of a gram per liter of chromium.

Example VI A bath similar to that of Example V was made up with tungstenas the metal brightener. bath had the following composition:

The

Grams per liter Zinc oxide (ZnO),

45 Sodium hydroxide (NaOH) 38 Sodium cyanide (NaCN) Tungsten trioxide(WOa) 5 r Thiourea 10 Very good results were obtained with this bath.

The bath contained about four grams per liter of tungsten in the form ofa soluble compound.

Example v11 A cyanide-zinc bath with about one and seventenths grams perliter of uranium as the brightener is given:

Grams per liter Zinc oxide (ZnO) 45 Sodium hydroxide (NaOH) 38 Sodiumcyanide (NaCN) 80 Uranium oxide (U308) 2 Example vm A bath of thefollowing composition illustrates the use ofmetals of group VIII series4 of the Periodic System, together with a metal of group VI sub-group 1.

Grams per liter Zinc oxide (ZnO) 45 Sodium hydroxide (NaOH) 38 Sodiumcyanide (NaCN) Molybdenum trioxide (M003) 2 Cobalt sulfate (C0SO4-7H2O)1 Phenylthiourea 20 Very good results were obtained by the use of thisbath. The thiourea addition agent may be omitted, if desired, or anotherorganic addition agent may be used in lieu thereof. It.is noted that themolybdenum trioxide introduced about one and three-tenths grams perliter of molybdenum, and the cobalt sulfate about twenty-one hundredthsgram per liter of cobalt.

Example IX l The following cyanide-zinc bath shows the use of an ironbrightener to solubilize manganese:

Grams per liter The bath contained about two and seven-tenths grams perliter of moylbdenum, about sixty-six hundredths of a gram per liter ofiron, and about one gram per liter of manganese.

complex which was made by adding sodium cyanide to manganese sulfate.

It will be noted that in Example IV, sodium cyanide is used in about theknown proportion for baths of this character. However, in the bath ofExample IX, for instance, a considerably larger proportion of sodiumcyanide is used. I have found that electrodeposits of improved color areobtained with baths of this type if no less than about ninety grams perliter of sodium cyanide is employed. Generally I prefer to use betweenabout ninety and one-hundred and thirty grams per liter of sodiumcyanide with baths having about the indicated amounts of zinc oxide andsodium hydroxide, while, more specifically, I prefer to use aboutone-hundred grams per liter, say ninety-five to one-hundred and fivegrams per liter. It will, of course, be understood that the baths'may bemore dilute with a consequent reduction in the number of grams per literof the constituents;

While the metals of group VI sub-group 1 are shown in about an optimumamount in each of the above examples, the quantities used may be greatlyvaried. Generally speaking, the metals of group VI sub-group 1 should beused in substantial amount. The upper limit on the quantitles of thesemetals is largely determined by economic considerations, because, inview of their high price, it would not at the. present time becommercially feasible to employ very large amounts of these relativelyexpensive metals. More specifically considered, the metals of group VIsub-group 1 should be used in amounts not substantially less than aboutone hundredth gram per liter, and no more than about forty grams perliter can economically be used. Molybdenum has proved the mostsatisfactory of the metals of this group, and I generally prefer to usefrom about one-fourth to twenty-five grams per liter iii) of thismet-a1.

amounts from about one to twelve grams per liter.

It will be understood that, while reference is made.

metals of group VI sub-group 1 is required for.

optimum results, as will appear from a consideration of the examples.Generally, I prefer to use no less than about five hundredths of a gramof ametal of group VII sub-group 1, and the metals of group VIII series4, similarly, should be used in amounts not substantially smaller thanlive It is noted, that the manganese was added in the form of a positionwere made up:

More specifically, the best results I are obtained when molybdenum isused in hundredths of a gram per liter. It will be readily understoodthat the optimum amounts of the various constituents of baths madeaccording to my invention can best be determined for each particularbath by a few simple trials.

The amounts of thiourea shown in the above To replace molybdenum removedfrom the bath, zinc-molybdenum alloys of the following com- Per cent 1.Molybdenum 0.0087 2. Molybdenum 0.03 i 3. Molybdenum 0.095 4. Molybdenuma 0.11 5. Molybdenum"; 0.22 6. Molybdenum 0.39 7. Molybdenum 0.55

The preferred anode alloy is number 3, with 0.095% of molybdenum.

Similar alloys can be used. of course, when other metal brighteningagents are employed. When two such brightening agents are used, it mayprove advantageous to use zinc alloyswhich contain substantially theproper amounts of each agent used in the bath.

The zinc deposits produced according to the processes of my inventionare characterized by the presence of a small but substantial amount of ametal oi group VI sub-group 1. Such deposits usually contain no lessthan about two thousandths of one per cent of such a metal. Moreparticularly, when molybdenum is used as a brightener according to mypreferred processes, the zinc deposits obtained usually contain fromfive hundredths to one per cent of molybdenum.

Deposits produced according to the processes of my invention may alsocontain other metals used as brighteners such as metals of group VIIsubgroup 1, for instance manganese.

While I have given certain specific proportions and have listedconditions in the foregoing, it will be understood that I do not intendto be limited thereby. the scope of my invention being set forth in thefollowing claims:

I claim:

1. A cyanide-zinc electroplating composition for plating a depositcomprised substantially of zinc, the composition containing ametal fromgroup VI sub-group 1 of the Periodic System in the form of a solublecompound.

2. A cyanide-zinc electroplating composition for plating a depositcomprised substantiallyof zinc, the composition containing a substantialamount of molybdenum in the form of a soluble compound.

3. A cyanide-zinc electroplating bath for plating a deposit comprisedsubstantially of zinc, the bath containing about one hundredth to fortygrams per liter of molybdenum in the form of a soluble compound.

4. A cyanide-zinc electroplating bath for plating a deposit comprisedsubstantially of nine,

. thiourea.

6. A cyanide-zinc electroplating bath for plat ing a deposit comprisedsubstantially of zinc, the bath containing a. substantial amount ofmolybdenum in the form of a soluble compound, and

thiourea.

7. A cyanide-zinc electroplating bath for plating a deposit comprisedsubstantially of zinc, the bath containing a substantial amount of ametal of group VI sub-group l of the Periodic System, V

and a substantial amount of a metal of group VII sub-group l of thePeriodic System, the metals being in the form of soluble compounds.

8. A cyanide-zinc electroplating bath for plating a deposit comprisedsubstantially of zinc, the bath containing a substantial amount of ametal of group VI sub-group 1 of the Periodic System, and a substantialamount oi. a metal of group VIII series 4 or the Periodic System, themetals being present in the form of soluble compounds.

9. A cyanide-zinc electroplating bath for plating a deposit comprisedsubstantially of zinc, the bath containing a substantial amount ofmolybdenum and a substantial amount of manganese.

10. In a process for "the electrodeposition of a deposit comprisedsubstantially oi! zinc, the step comprising depositing zinc from acyanide-zinc bath in the presence of a metal from group VI sub-group 1of the Periodic System in the form of a soluble compound.

11. In a process for the electrodeposition of a deposit comprisedsubstantially of zinc, the step comprising depositing zinc from acyanide-zinc bath in the presencefot a substantial amount of molybdenumin the form-of a soluble compound.

12. In a process for the electrodeposition-oi a deposit comprisedsubstantially of zinc, the step comprising depositing zinc from acyanide-zinc bath in the presence of a substantial amount'oi a metal ofgroup VI sub-group 1 of the Periodic System in the form of a solublecompound, and thiourea. t.

13. In a process for the electrodeposition 0! a deposit comprisedsubstantially of zinc, the step comprising depositing zinc from acyanide-zinc bath in the presence of a substantial amount of molybdenumand a substantial amount of manganese.

14. In a process for the electrodeposition of a deposit comprisedsubstantially of zinc, the step comprising depositing zinc from acyanide-zinc bath with an electric current introduced into the bath witha zinc alloy anode which contains a metal of group VI sub-group 1.

15. An electrodeposit comprised substantially of zinc and characterizedby the presence of a substantial amount of a metal of group VI subgroup1 of the Periodic System. v

16. A cyanide-zinc electroplating bath made up with materials comprisingabout ninety to one-hundred and thirty grams per liter of sodiumcyanide, a zinc compound and a soluble compound of a metal of group VIsub-group 1 of the Periodic System. I

1'7; In a process for the electrodeposition of a deposit comprisedsubstantially of zinc, the step comprising depositing zinc from acyanide-zinc bath in the presence of a substantial amount of a metal ofgroup VI sub-group 1 and a substantial amount of a metal of group VIIIseries 4 of the Periodic System, the metals being present in the form ofsoluble compounds.

18. In a process for the electrodeposition of a deposit comprisedsubstantially of zinc, the step comprisingdepositing zinc-irom acyanide-zinc bath in: the' presence of from about one-hundredth to fortygrams-per liter of molybdenum in the form oia soluble compound.

19. In a process for the electrodeposition of a deposit comprisedsubstantially of zinc, the step comprising depo siting zinc from acyanide-zinc bath in the presence of from about one-fourth totwenty-five grams per liter of molybdenum in the formpf a solublecompound.

LEON R. WESTBROOK.

